VOICE AND SPEECH 



619 



The larynx is covered internally by a mucous membrane continuous with that of 

 the trachea. It is lined with ciliated epithelium, except over the vocal cords, where 

 the epithelium is stratified. The two vocal cords, or thyro-arytenoid ligaments, con- 

 sist of elastic fibres which run from the middle of the inner angle of the thyroid cartilage 

 to be inserted into the anterior angle of the arytenoid cartilages. Their length in man 

 is about 15 mm., in woman about 11 mm. The cleft between them is known as the 

 glottis, or rima glottidis. 



Two ridges of mucous membrane above and 

 parallel to the vocal cords are the false vocal 

 cords (Fig. 310). Bstween the true and the false 

 vocal cords on each side is a recess known as the 

 ventricle of Morgagni. This ventricle permits 

 the free vibration of the vocal cords. The false 

 cords take no part in phonation, but help to keep 

 the true cords moistened by the secretion of the 

 numerous mucous glands with which they are 

 provided. The false cords are also used in hold- 

 ing the breath. For this purpose they function 

 in a similar manner to the mitral valve of the 

 heart. It is found that animals who need the 

 thorax to be fixed in order that they may climb 

 or strike have well developed false cords. The 

 position and tension of the vocal cords are 

 determined by the action of the 'intrinsic 

 muscles of the larynx. The part taken by the 

 various muscles in each movement cannot be 

 directly ascertained. We can in most cases study 

 only the direction of the fibres, and judge, 

 from this direction and consequent isolated 

 action of the muscles, the part taken by any 

 given muscle in the 'production of voice. The 

 chief muscles (Fig. 311) are as follows: 



(1) The crico -thyroid muscle is a short trian- 

 gular muscle attached below to the cricoid 

 cartilage and above to the inferior border of the 

 thyroid cartilage ; the fibres pass from below 

 upwards and backwards. When this muscle 

 contracts, the cricoid cartilage is drawn up 

 under the anterior part of the thyroid cartilage, 

 so that its broad expansion behind, with the 

 arytenoid cartilages, is drawn downwards and 



backwards, thus putting the vocal cords on the stretch. This muscle is probably the 

 most important in determining the tension of the vocal cord. 



(2) The posterior crico -arytenoid muscle arises from a broad depression on the corre- 

 sponding half of the posterior surface of the cricoid cartilage. It passes upwards and out- 

 wards, its fibres con verging, to be inserted into the outer angle of the arytenoid cartilage. 

 These muscles rotate the outer angle of the arytenoid cartilages backwards and inwards. 

 They thus cause a movement outwards of the anterior angles, so that the glottis is widened. 

 During every act of inspiration there is a widening of the glottis, which is probably 

 effected by contraction of these muscles. If they are paralysed the vocal cords are 

 approximated and tend to come together during inspiration, so that dyspnoea may be 

 produced. 



(3) The lateral crico -arytenoid muscle arises from the upper border of the cricoid 

 cartilage and passes backwards to be inserted into the muscular process of the arytenoid 

 cartilage. These muscles when they contract pull the muscular process of the arytenoid 



FIG. 310. Anterior half of the larynx, 

 seen from behind. The section on 

 the right side is somewhat in front 

 of the left side. 



e, epiglottis ; e' ', cushion of epi- 

 glottis ; t, thyroid cartilage ; s, ', 

 ventricle of larynx ; h, great cornu 

 of hyoid bone ; t a, thyro-arytenoid 

 muscle ; vl, vocal cords Above the 

 ventricles are the false vocal cords, 

 r, first ring of trachea. 



(A. THOMSON.) 



